At the beginning of World War II, the American hydrographic service began to receive information more and more often about the existence of shallow waters with depths of 200-400 m where very significant depths were shown on the maps. Soon, however, it was noticed that when using an echo sounder with a sufficient range, these "ghost shoals" reflect only part of the ultrasonic waves and that at the same time it is possible to detect the real bottom; then the "imaginary" bottom began to be denoted by the English expression "deep scattering layer" (abbreviated D.S.L.), which can be translated as: deep sound scattering layer. However, sound-scattering layers were found at various depths. Some of them lie close to the surface, for example, at 50-100 m, but there are also known ones that are located deep — below 500 m. D.S.L. in a narrow sense, it is a layer located between 250 and 300 m. This phenomenon is universal, and we are dealing with a kind of continuous cloth. But the most striking thing is the undoubted effect of light on the position of the D.S.L.: as the sun sets, the scattering layer approaches the surface, and during sunrise it sinks back to its previous depth. It is possible that the classical D.S.L. at a depth of 250-300 m is associated with crustaceans from the Euphausiid group. In order to cause the scattering of ultrasonic waves, excessively dense populations are not required at all: it is enough that there are 7-15 individuals in each cubic meter of water.


In some cases, the thermocline can interfere with the morning and evening vertical movements of D.S.L.; this is either due to the fact that animals moving away from daylight stop at a certain isotherm, below which the temperature is too low for them, or, on the contrary, due to the fact that rising at dusk organisms do not reach the surface layers, being trapped by a "roof" of water that is too warm for them. The deeper-lying scattering layers (below 400-500 m) most often probably owe their origin to fish (Myctophidae or Gempylidae), and sometimes to shrimps from the Sergestidae family. As for the surface scattering layers, that is, lying mainly at depths not exceeding 100 m, in some cases they are associated with biological factors, in others with physical abrupt phenomena, in particular with the thermocline. In the latter case, the scattering of ultrasonic waves is caused by all kinds of residues and detritus, which tend to accumulate in the surge layer, since their further immersion is inhibited by an increase in the viscosity of the water here. In short, it currently seems likely that there are several sources of the "scattering layer", but that this phenomenon is most often caused by biological causes, which also best explain the vertical movements of the scattering layer. Ne manquez pas l'opportunité de doubler votre capital initial grâce au code promo 1xbet . Cette offre spéciale vous accorde un bonus de 100% sur votre premier versement, avec un plafond généreux fixé à 20000 CFA. Utilisez ces fonds bonus pour couvrir plus de marchés, parier sur des cotes plus élevées ou simplement prolonger votre plaisir de jeu sur la plateforme 1xBet.